The head of MI5, Eliza Manningham-Buller, warned last night of the possibility of terrorist attacks on the food and chemical industries and said it was clear that the threat from extreme Islamists would be "with us for a long time".

In a rare public speech, the MI5 director general told the City of London police that well-established security advice was in place to protect communications, transport, water, fuel and energy supplies. "But the changed nature of the threat has meant that we need to extend that advice to new sectors such as the chemical and food industry, which, today, may present an attractive target for terrorists."

Threats of chemical, biological, radiological and suicide attacks required new responses which were not up to the government alone, she added. MI5 was strengthening its ties with commercial organisations but industry and even the public had to take greater responsibility for their security.

The scale of what she called the "high level of threat" from Islamist terrorism had become more apparent as the volume of intelligence increased. Whitehall's new joint terrorism analysis centre dealt every week on average with 100 pieces of threat intelligence - plans or intentions to attack.

But, the director general warned that the initiative rests with the terrorists. "The timing of any attack is of their choosing and for them patience is part of the struggle."

There would be more alerts, she said, adding that there had to be a balance between being alarmist and too optimistic.

Individuals sympathetic to the aims of the al-Qaida terrorist network blended into society, and lived normal, routine lives until called upon for specific tasks by a part of the network, she said.

"Some of those individuals," she said, without estimating how many, "are in the UK."

It was MI5's highest priority, she said, to gather intelligence on suspects in Britain. Counter-terrorism now accounts for 70% of MI5's resources and its 2,000 officers.

Nearly all of its inquiries had an international dimension.

She also said intelligence was often a matter of interpretation. "Some intelligence is aspirational not substantial, some designed to mislead, some accurate in parts but wrong in others. Analysing it rarely leads to certainty."

She chastised those who described the ordering of troops to Heathrow early this year as a cynical government manoeuvre to prepare Britain for war against Iraq.